Optical limiters are devices designed to have high transmittance for low-level light inputs and low transmittance for high power light inputs. Since the development of the first lasers, passive optical limiters have been researched and concepts have been tested to protect optical sensors against laser peak-power induced damage. The first optical limiters for CW lasers were based on thermal lensing in absorbing bulk liquids, i.e., local heating in an imaging system reduced the index of refraction of the limiting material, causing “thermal blooming” and resulting in a beam that was no longer focused. Other methods have been suggested for limiting pulsed laser sources such as reverse saturable absorption, two-photon and free carrier absorption, self-focusing, nonlinear refraction and induced scattering. The limiting device itself must also possess a high threshold against damage, and not get into a state where it is “bleached-out” by high power or turning transparent by high power.
Communications and other systems in medical, industrial, and remote sensing applications, may handle relatively high optical powers, from microwatts up to several watts, in single fibers or waveguides. With high intensities (power per unit area) introduced into these systems, many thin film coatings, optical adhesives, and even bulk materials, are exposed to light intensity beyond their damage thresholds. Another problem is laser safety, wherein there are well-defined upper power limits allowed to be emitted from fibers into the open air. These two issues call for a passive device that will limit the amount of energy propagating in a fiber/waveguide to the allowed level.
There have been many attempts to realize optical limiters, mainly for high power laser radiation, high power pulsed radiation, and eye safety devices. The techniques used in these devices have been mainly:                1. Thermal change of the index of refraction n, in liquids having negative dn/dT, for defocusing the light beam, e.g., in an imaging system.        2. Self-focusing or self-defocusing, due to high electric field-induced index of refraction n change, through the third order susceptibility term of the optical material, here n=n0+n2E2 where n0 is the index of refraction at zero electric field (no light), n2 is the non-linear index change and E is the electric field strength of the light beam.        3. Colloidal suspensions of absorbers in both polar and non-polar solvents that limit by the creation of plasma and induced scattering.        4. Non-linear nanostructures and materials, that are highly scattering when exposed to high light intensities. These materials show low thresholds due to field enhancement and nano-particle dimensions.        
Both No. 1 and 2 of the above-mentioned techniques require very energetic laser beams or light intensities to produce a meaningful limitation. In the first technique, the volumes of liquid to be heated are large and need high powers. Another problem with this technique is that the liquid is not a good optical medium and distorts the beam. In the second technique, the n2 coefficient is very small for usable materials and requires very high electric fields.
In the third technique, the use of liquids is problematic for most applications. In a communications system, for instance, the use of liquids in a passive device causes noise and distortion from turbulence of the liquid in the optical path. Other problems reported using the colloidal liquid as an optical-limiting medium include aging either by disappearance of the active material or the formation of flocs of loosely bound large particles that break up only after ultrasonic deflocculation. Some work has been done on using liquid crystals as limiting material, mainly for high power pulses, but these materials cause noise and distortion worse than ordinary liquids due to directional fluctuations.
The use of nanostructures having various nanoparticles in a solid matrix for limiting devices is described in Israeli patent 147554 and EP 1467239 A2 (uses nanostructures for limiting through scattering).
There is a constant need for low-threshold limiters, mainly for protection from strong light sources.